1. Field of the Invention
The present invention relates to a magnetic recording medium which records information using magnetization directions as well as to an information reproducing apparatus which is equipped with the magnetic recording medium and reproduces information according to the magnetization at various points on the magnetic recording medium.
2. Description of the Related Art
Recently, with the spread of computers, large volumes of information have been handled on a daily basis. Apparatus which handle such large volumes of information include a hard disk drive (HDD). The HDD incorporates amagnetic disk, i.e., a disk-shaped magnetic recording medium on which information is recorded, and a magnetic head which records and reproduces information on/from the magnetic disk.
The magnetic disk has a magnetic layer of ferromagnetic material formed on a non-magnetic substrate. The magnetic layer is divided into minute areas and information is carried by magnetization directions of the minute areas. A protective layer of carbon or the like is formed on the magnetic layer and a lubricating film of perfluoropolyether (PFPE) is formed on the protective layer.
FIG. 1 is a diagram schematically showing a lubricating film of a conventional magnetic disk.
A magnetic disk which has the lubricating film 900 shown in FIG. 1 has a through-hole in the center and carries information in an outer zone outside a predetermined inner zone around the through-hole. Hereinafter, the outer zone will be referred to as a recording zone. A hub rotated by a drive motor is mounted in the through-hole in the magnetic disk, and the magnetic disk rotates around the dashed line at the left of FIG. 1 during operation of the HDD. A magnetic head incorporated in the HDD is placed in proximity to the magnetic disk and a floating surface comes into contact with and slides over a surface of the magnetic disk when the magnetic disk starts or stops to rotate, but it remains floating over the magnetic disk during rotation of the magnetic disk.
The lubricating film 900 plays a role in reducing wear caused by sliding of the magnetic head and in preventing the information carried by the magnetic layer from being destroyed due to contact with the magnetic head (head crash). The lubricating film 900 shown in FIG. 1 is formed by applying liquid PFPE uniformly to the surface of the protective film with an accuracy on the order of 0.1 nm (Å) and solidifying it subsequently by heat treatment or ultraviolet irradiation. Although the lower part of the lubricating film 900 thus formed is in close contact with the protective film, its upper part moves toward the outer periphery (outer side) of the magnetic disk due to centrifugal force generated by the rotation of the magnetic disk. This movement is sometimes referred to as a migration phenomenon. In FIG. 1, the lower part of the lubricating film 900 which is in intimate contact with the protective film is shown as a bonding layer 910 while the upper part which is moved outward by centrifugal force is shown as a free layer 920. The free layer 920 has the self-repairing effect of filling scratches produced by collisions with the magnetic head, by being moved by centrifugal force. The thickness of the bonding layer 910 in FIG. 1 is uniform in the radial direction of the magnetic disk and so is the thickness of the free layer 920.
Also, various proposals have been made about lubricating films (see, for example, Japanese Patent Laid-Open No. 2000-268358, National Publication of International Patent Application No. 2002-516454, Japanese Patent Laid-Open Nos. 62-226423, and 2001-134924). Japanese Patent Laid-Open No. 2000-268358 discloses a lubricating film in which a bonding layer is thicker in an inner zone (CSS zone) around a through-hole in a magnetic disk than in a recording zone outside the CSS zone. Japanese Patent Laid-Open No. 2002-516454 discloses a lubricating film which is thinner in an inner zone (landing zone) than in a recording zone outside the landing zone. Japanese Patent Laid-Open No. 62-226423 discloses a lubricating film whose thickness is at its maximum in the innermost zone and decreases with increasing distance from the center. Japanese Patent Laid-Open No. 2001-134924 discloses a technique of reducing the thickness of a free layer.
However, when the magnetic disk rotates for a long time, the free layer 920 on the bonding layer 910 which has a uniform thickness (shown in FIG. 1) or the free layer on the bonding layer which becomes thicker with decreasing distance from the center (disclosed in Japanese Patent Laid-Open No. 2000-268358) tends to lean to the outer side due to the migration phenomenon.
FIG. 2 is a graph showing film thickness of the lubricating film shown in FIG. 1 in the radial direction of a magnetic disk after the magnetic disk with the lubricating film is rotated continuously at 10,000 rpm in a high-temperature environment for 30 days.
The horizontal axis of the graph shown in FIG. 2 represents the radial distance (mm) from the center of the magnetic disk while the vertical axis represents the film thickness (Å) of the lubricating film. The graph in FIG. 2 shows the film thickness of the lubricating film in a recording zone which carries information. It can be seen from FIG. 2 that the lubricating film becomes thicker with increasing distance from the center of the magnetic disk. This is believed to be because the free layer 920 is moved to the outer side due to the migration phenomenon.
In the case of the lubricating film whose thickness in the landing zone is increased, i.e., the lubricating film disclosed in National Publication of International Patent Application No. 2002-516454, although the free layer is thicker in the inner landing zone than in the outer recording zone, the thickness of the free layer in the recording zone is uniform in the radial direction and thus, that part of the free layer which is located in the recording zone tends to lean to the outer side due to the migration phenomenon.
If the free layer leans to the outer side, that part of the free layer which is on the outer side becomes thicker than that part of the free layer which is on the inner side and can easily hit the magnetic head floating over it, damaging or contaminating the magnetic head. The free layer leaning to the outer side can eventually scatter out of the magnetic disk due to centrifugal force. If the free layer is removed from the top of the bonding layer, the self-repairing effect can no longer be realized. Besides, the magnetic head touching the bonding layer is liable to wear and damage. Furthermore, head crashes can occur frequently. Thus, it is desired to minimize the possibility of the free layer becoming thicker near the outer periphery of the recording zone than near the inner periphery.
In the case of the lubricating film which becomes thinner with increasing distance from the center of the magnetic disk, i.e., the lubricating film disclosed in Japanese Patent Laid-Open No. 62-226423, the bonding layer and free layer are not distinguished, and thus, it is not possible to discuss the migration phenomenon. Even if the thickness of the free layer is decreased using the technique disclosed in Japanese Patent Laid-Open No. 2001-134924, the tendency for the free layer to lean to the outer side due to the migration phenomenon remains unchanged. Besides, if the thickness of the free layer is reduced too much, the free layer tends to be removed from the top of the bonding layer, again resulting in a problem described above.